1. Field of the Invention
The present invention relates to a multilevel image recording method for recording multilevel images by changing the recorded surface areas of a plurality of dots forming one pixel.
2. Description of the Related Art
A multilevel image recording method is known in which the density of one pixel is expressed by recorded surface areas of a plurality of dots by using thermal heads, in which a plurality of heat-generating elements are aligned in a direction (the main scanning direction) orthogonal to the direction (subscanning direction) in which the recording paper is conveyed, as a means for heat-sensitizing/recording a multilevel image.
In this method, the density of one pixel is expressed by distributing the density of the pixel into a plurality of dots forming the pixel. The density of the pixel is expressed by the total sum of the densities of the respective dots.
In accordance with the above-described method, the plurality of dots each express a gradation. Therefore, as compared with a case in which the density of one pixel is expressed by one dot, the number of gradations which can be expressed is increased and the density can be expressed linearly. An example of a combination of plural dots forming one pixel is two dots which are adjacent in the main scanning direction.
Recently, there has been a demand for expression of images of a higher number of gradations (high resolution images). When images of a higher number of gradations are expressed by using the above-described method, in order to increase the number of gradations which can be expressed, the number of dots forming one pixel may be increased. However, because the gradation of the image depends on the dot pitch, when images of a high number of gradations are to be expressed, the heat-generating bodies of the thermal heads must be disposed at a narrow pitch.
The coloring density of the recording paper depends on the energy supplied to the recording paper from the thermal heads. The recording paper is colored when the energy supplied from the thermal heads exceeds a coloring threshold value of the recording paper.
Usually, when an image of about 256 gradations is recorded at a low resolution of about 400 dpi, the pitch of the heat-generating bodies of the thermal heads is sufficiently ensured as compared with the number of gradations. As a result, even when a shadow region, in which the pixels have large recording surface areas, is recorded, because the space between adjacent pixels is sufficiently ensured, adjacent pixels do not contact one another.
However, when an image is recorded at a high resolution of about 600 dpi, the pitch of the heat-generating bodies is narrow as compared to the number of gradations, and the space between adjacent pixels is narrow. As a result, adjacent pixels may contact one another in shadow regions, in which the recorded surface areas of the pixels are large, and even in highlight regions in which the recorded surface areas of the pixels are small.
Conventionally, densities which are distributed to a plurality of dots forming one pixel are set per pixel. As a result, for an image which is to be reproduced, the relationship of the lightness and darkness of the density of each dot forming one pixel differs per pixel.
FIG. 3 is a partially expanded view of an image reproduced by a conventional means, and illustrates a case in which the image is reproduced by using two dots for one pixel. The density of two dots forming one pixel is illustrated by surface area gradation in FIG. 3. However, in the encircled region in FIG. 3, the relationship of lightness and darkness of the densities of two dots forming one pixel is opposite that of other regions.
When the relationship of lightness and darkness of the densities is opposite, the directions of pixels P become non-uniform, and adjacent pixels contact each other such that the space between adjacent pixels is eliminated. If the directions of the pixels P become non-uniform, the image gives off the impression (feeling) of roughness. Further, if the space between adjacent pixels is eliminated, jumps in gradation become marked particularly in highlight regions, and accordingly, a linear gradation characteristic cannot be obtained.